Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head "flies" over a surface of the disc. The transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back.
Over the years, storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the transducing head must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the transducing head. This is known as a "crash" and can damage the transducing head and surface of the storage disc resulting in loss of data.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor, because of low damping. Thus, there has been a search for alternative bearing assemblies for use with high density magnetic storage discs.
One alternative bearing design which has been investigated is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. In addition to air, typical lubricants include oil or ferromagnetic fluids.
Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Moreover, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeatable runout.
However, hydrodynamic bearings themselves suffer from disadvantages, including a low stiffness-to-power ratio. These problems lead to a high sensitivity of the bearing to external loads or shock.
A desirable solution to this problem would be to have the spindle motor attached to both the base and the top cover of the disc drive housing. This would increase overall drive performance. A motor attached at both ends is significantly stiffer than one held by only one end.
In order to have top cover attachment, the motor bearing would need to be open on both ends. Opening a hydrodynamic bearing type motor bearing at both ends greatly increases the risk of oil or fluid leakage out of the hydrodynamic bearing. This leakage is caused among other things by small differences in flow rate created by differing pumping pressures in the bearing. If all of the pressure gradients within the bearing are not carefully balanced, a net pressure rise toward one or both ends will force the fluid out through the capillary seal, thus creating a huge fluid wetted surface.
Another problem which follows from leaving the hydrodynamic bearing open at both ends is that more of the bearing is exposed to the atmosphere, increasing the possibility of evaporation of the fluid over the long term. Such a fluid loss would have a serious effect on the stiffness of the bearing as well as overall reliability of the drive.
A related problem is that it is desirable, in terms of the stiffness of the bearing and minimization of evaporation, to use a higher viscosity fluid. However, such higher viscosity fluids can create a drag effect which increases the power required for the motor.
Thus, the need exists for a new approach to the specification of hydrodynamic bearing fluids to optimize the motor performance and stiffness while diminishing evaporation.